1. Field of the Invention
This invention relates to a high-temperature, non-catalytic, infrared heater having a burner and a re-radiating surface proportioned to operate the heater at approximately 600xc2x0 F. to 1400xc2x0 F.
2. Description of the Prior Art
Thermoforming is a process which uses heat and pressure and/or vacuum to form parts from an extruded sheet of thermoplastic. In the process of thermoforming, plastic is drawn from large rolls, heated to its softening temperature, and then formed into a desired shape using an aluminum forming tool. The plastic is then cut into individual containers, stacked, inspected, counted, boxed and shipped. The heating section uses infrared heaters to soften the plastic sheet to forming temperature.
The thermoforming industry currently consists of over 500 manufacturers representing 6,300 manufacturing lines. Of these, 95% utilize electric infrared heaters while the remainder are gas fired.
Thermoforming machine manufacturers and end-users have identified that the electric energy cost to heat the plastic sheet accounts for 35% to 50% of the cost of the end product. The ability to use natural gas as the primary source of heating energy could reduce this number to less than 10% due to the 3:1 cost advantage of natural gas. The primary reason for not using gas-fired infrared is due to operating temperature and control limitations of available gas heaters. The optimum operating temperature of a heater used in the thermoforming process is 600xc2x0 F. to 1200xc2x0 F. Gas catalytic heaters operate between 400xc2x0 F. to 950xc2x0 F. or gas ceramic heaters operate between 1400xc2x0 F. to 2000xc2x0 F. Therefore, a gas fired infrared heater that operates between 600xc2x0 F. to 1200xc2x0 F. would offer the thermoforming industry a cost-effective product to reduce operating cost while maintaining part quality.
Three types of heaters are currently in use in the thermoforming industry including: (1) electric heaters that operate between 600xc2x0 and 1200xc2x0 F., have a power density of 30 watt/in2 and an overall efficiency  greater than 40%; (2) gas catalytic heaters that operate  less than 950xc2x0 F., have power density=10 watt/in2 and an overall efficiency  less than 30%; and (3) gas ceramic heaters that operate  greater than 1400xc2x0 F., have a power density greater than 50 watt/in2 and an overall efficiency  less than 30%.
Power density is defined as the total energy input into the heater divided by the surface area. This number is often confused with and is always higher than the actual radiant heat flux delivered to the plastic (i.e. 30 watt/in2 radiant heat flux vs. 50 watt/in2 power density). The overall efficiency is defined as the energy absorbed by the plastic sheet divided by the total energy input into the heater. It is therefore necessary and a desired object of this invention to improve the efficiency of gas based heaters.
It is one object of this invention to provide an optimum heater for a radiant heat flux of 5 to 30 watt/in2 with a heater surface temperature between 400xc2x0 F. and 2200xc2x0 and more preferably 600xc2x0 F. and 1400xc2x0 F. to promote efficient and uniform vertical heating of a plastic sheet.
It is another object of this invention to provide a heater that increases the overall efficiency of the heating process to greater than 40% by minimizing exhaust temperature and/or recovering waste heat of the IR heater.
It is another object of this invention to provide a heater that minimizes the power density of the heater to less than 50 watt/in2 by increasing the conversion of fuel energy to radiant energy.
It is a further object of this invention to provide a heater having a surface area of approximate dimensions that allow for easy retrofit to existing machines.
It is yet another object of this invention to provide a heater that promotes temperature uniformity of the heat to better than xc2x110xc2x0 F. to maintain consistent horizontal heating.
A heater according to a preferred embodiment of this invention comprises a housing preferably including a bottom and at least one and preferably four sides and lined with a refractory material. A burner is preferably positioned within the housing which operates in a preferred range of between approximately 1400xc2x0 F. and 2200xc2x0 F.
A re-radiating surface have a re-radiating surface area is preferably positioned above the burner. The re-radiating surface may comprise a mesh of a variable mesh size and/or a variable mesh configuration across the re-radiating surface area. The re-radiating surface area according to one experiment may be greater than the burner surface area by approximately five times.
To improve the overall efficiency of the gas based heaters it is necessary to match the wavelengths of the heater surface to that of the plastic. In a preferred embodiment of this invention, the heater is operated at a surface temperature of approximately 1200xc2x0 F. According to this invention, another effective manner of improving the overall efficiency of gas-based heaters is to improve the conversion of gas-based energy to radiant energy. This is accomplished by minimizing heater exhaust temperatures with enhanced heat transfer or heat recovery devices. Both of these methods will increase the overall efficiency of the heater and reduce the power density of the heater. The radiant heat flux will remain the same assuming the heater surface temperature is maintained at 1200xc2x0 F.
The variable area design uses a burner source of smaller surface area (Ah) than a re-radiating surface (Ar) to convert combustion energy into usable radiant power. The heater is an infrared heater that operates at surface temperatures greater than 1400xc2x0 F. and is of a premix design. Its surface may be shaped from concave to flat to convex and may be of any geometric shape, including square, circle, etc. The burner radiates heat from its surface to the surface area of the re-radiating surface. In addition, high temperature exhaust gases emanate from the surface of the burner and flow to the re-radiating surface. The re-radiating surface is heated by radiation from the burner and by convection from the recirculating gases in the box. This energy, in turn, is transferred by conduction within the re-radiating surface to the re-radiating surface""s surface which in turn radiates heat to the plastic sheet.
According to a preferred embodiment of this invention, the burner surface area (Ah) is smaller than the re-radiating surface area (Ar). This design diffuses the energy to the re-radiating surface and allows the surface to operate at a temperature less than 1400xc2x0 F. In addition, the burner is preferably enclosed in a refractory lined box that receives heat from the radiator and reflects this heat to the re-radiating surface. By efficiently convecting the exhaust energy to radiant energy, the exhaust temperature exiting the surface of the re-radiating surface is near a theoretical minimum. The exhaust temperature is slightly higher than the re-radiating surface temperature.
The areas are preferably designed such that when the burner is at 1400xc2x0 F., the re-radiating surface is at 600xc2x0 F., and when the burner is at 2200xc2x0 F., the re-radiating surface is at 1400xc2x0 F. The re-radiating nature of the re-radiating surface will provide heat back to the burner and allow it to operate at lower inputs than without the re-radiating surface. This heat re-radiating back to the burner keeps the burner""s surface at a higher temperature and allows combustion to occur at the auto-ignition temperature of 1400xc2x0 F. while at lower power inputs to the heater.
The re-radiating surface will have a mesh of varying size and a mesh of variable design at different locations on the re-radiating surface. It may also be designed with perforated plates or rods of metallic, refractory, or ceramic material.